1. Field of the Invention
The present invention is broadly concerned with decorative architectural panels formed of heat treated titanium, as well as methods of forming such panels and improved fixturing permitting the heat treatment of multiple panels without warpage or distortion thereof. More particularly, the invention is concerned with such panels which are subjected to multiple, controlled heating and cooling steps so as to recrystallize the titanium surface to give a pleasing, faceted appearance; preferably, the relatively large architectural panels are individually framed before heat treatment so as to resist unwanted edge distortions.
2. Description of the Prior Art
Titanium is a relatively light, silver-gray metal with a specific gravity of 0.163 lb/in3. Pure titanium has a high melting point (3035xc2x0 F.) and a lower coefficient of expansion and lower thermal conductivity than either steel or aluminum alloys. Its modulus of elasticity (1.1xc3x971011 Pa) is midway between that of steel and aluminum.
Titanium is allotropic, and up to a temperature of 1625xc2x0 F., titanium atoms are in a hexagonal close-packed alpha crystal array. When titanium is heated above the transition temperature of 1625xc2x0 F., the atoms rearrange themselves into a body-centered cubic beta structure.
Commercially pure titanium is similar in physical properties to steel. However, by addition of other elements, the resultant titanium alloys are converted to materials having unique characteristics, including high strength and stiffness, corrosion resistance and usable ductility. The type and quantity of alloy addition determines the mechanical and, to some extent, the physical properties of titanium.
Commercially pure titanium and its alloys are used in the aerospace industry and in other contexts where corrosion resistance is required. Titanium""s corrosion resistance is based upon its reactive nature, i.e, it has the ability to form, upon exposure to the atmosphere, a tight, tenacious oxide film that is resistant to a wide variety of media which would corrode other metals. Thus, titanium is resistant to chlorides and oxidizing agents such as nitric acid, and is immune to environmental corrosion.
It is known that heat treatment of relatively small pieces of substantially pure titanium can create surface changes giving a pleasing, decorative, faceted appearance. For example, U.K. Patent No. 1,175,355 describes the surface treatment of titanium, in the context of decoration of deep drawn thimbles. In this process, the titanium objects are heated either under vacuum or in an inert gaseous atmosphere at a temperature of 900-1200xc2x0 C. for at least five minutes to cause grain enlargement and a faceted surface effect. Thereafter, the heat-treated titanium is subjected to an anodizing process. Swiss Patent No. 513,012 is also directed to the heat treatment of small titanium objects.
While surface decoration of such small titanium items is known, no processes have been developed for surface recrystallization of large sheet-like members such as architectural panels. Direct adoption of the prior art techniques described above is entirely unsatisfactory, owing to the fact that the heat treatment tends to substantially warp the larger panels to the point that they are rendered unusable.
There is accordingly a need in the art for improved processes and products whereby large sheet-type architectural and similar panels can be provided. Such decorative panels could be used as the facia cladding of buildings and other structures, to provide not only a pleasing aesthetic appearance, but also to give a highly durable, corrosion resistant exterior.
The present invention overcomes the problems outlined above and provides relatively large architectural panels or sheets which are designed for, e.g., attachment to the exterior surfaces of buildings or other structures; the panels are treated to give various stages of recrystallization to thereby create visually impressive aesthetic designs. Broadly speaking, such panels are generally quadrate in configuration and are formed of substantially pure (normally at least 99% pure) titanium. Moreover, they have a length or width dimension of at least about 3 inches, and preferably substantially larger (on the order of at least about 24 inches), with at least one face of the panel being heat-recrystallized and having an oxidation coating over the recrystallized face. Under certain processing conditions, the panels may also assume an undulating shape which further increases the aesthetic effect.
In terms of the heat treatment method, it has been found that the architectural panels must be subjected to multiple, controlled oven heating steps with intermediate cooling between the heating steps so as to effect the desired grain growth and recrystallization of surface portions of the panel. During such multiple heating steps, at least a portion of the circumscribing margin of the panel is restrained, preferably through the use of a frame disposed substantially around the margin to inhibit moving thereof during heating. The frame normally includes a plurality of interconnected frame members cooperatively extending about substantially the entirety of the panel margin, with the frame members being formed of a material different than titanium and preferably selected from the group consisting of high temperature ceramics and molybdenum. In order to further rigidify the panel and frame assembly, stiffening elements may be inserted proximal to the corners of the panel, preferably adjacent the rear surface thereof.
The heating steps are preferably carried out under vacuum conditions typically on the order of 10xe2x88x923 to 10xe2x88x925 torr. The particular heating regimen employed is variable depending upon the size of the panel and the desired surface decoration. Generally speaking though, the multiple heating steps involve relatively rapid heating up to a maximum temperature range above the transition temperature of the titanium, whereupon this maximum temperature range is maintained for a period of time. Where two heating steps are employed, the second maximum temperature range is normally somewhat lower than the first maximum temperature range, but the second range is maintained for a substantially longer period of time as compared with the first time period.
Intermediate cooling on the other hand preferably includes the step of injecting an inert cooling gas into the oven, with argon being very suitable for this purpose. After a minimum temperature range is reached using inert gas cooling, the gas is removed and vacuum conditions reestablished for the next heating step.
After the recrystallization multiple heating steps are concluded, the panels may then be oxidized in air if an interference color is desired. Different time-temperature heating in air produces different types and intensities of coloration on the panels, which can be controlled for predetermined effect.
During fabrication, individual panels are first framed and are then suspended in spaced relationship from each other using a graphite and molybdenum hanger assembly. The entire hanger assembly with installed framed panels is then placed within a heating oven for recrystallization heating and oxidation.